Okay, an expired kit is giving you useless information. You can't trust any of it. And with API, the Nitrate reagents have a history of expiring before the rest of the reagents in a master kit do so you really want to get your water checked out by another source and also purchase new reagents.
Here's the bottom line tho, if you had ammonia and now have nitrates that did not come from your source water, you have completed your cycling. If you only have the 1 fish in the tank when it cycled, there is only enough bacteria bed to support what little ammonia that 1 fish is producing, proceed very slowly adding new stock because everything you add now will cause a "mini cycle" where it's going to add ammonia but the bacteria bed will grow and should keep it in check if you don't overload it. If you overload it, you will have spikes of ammonia and nitrites which will not do your new stock any favors. If you dilute it with water changes, your bacteria bed won't grow enough to support it right off and you will be doing water changes constantly. If you add slowly, it will grow continually to catch up with the new load. The good news is that once established, the bacteria bed grows rapidly so most often, you won't see the spikes unless you have 24 hour monitoring for it. In an overload, you could have spikes high enough to kill your fish, the bacteria bed catches up to the load before you test your water and you test out as everything in order. ( This is what 24 hour monitoring can show you. :^0 Just so you know.
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I would definitely have an outside source test your water then you test again with your new test reagents to confirm the readings.
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